Planar fluorescent lamps are useful in many applications, including backlights for displays such as liquid crystal displays. A common weakness in such fluorescent lamps is their limited illumination range.
Planar fluorescent lamps typically utilize an electric plasma discharge through a low pressure mercury vapor and buffer gas to produce ultraviolet radiation. The ultraviolet radiation strikes a fluorescent material which converts the ultraviolet radiation to visible light. To produce the low pressure plasma discharge, such lamps typically require a substantial minimum energy input. If the lamps are driven below the minimum energy input, the plasma discharge may not be formed, or may be highly non-uniform. Moreover, the efficiencies of such lamps can be degraded substantially at low level operation. To improve uniformity and efficiency, such lamps typically must be driven well above their minimum energy input levels so that a complete, uniform plasma discharge can be formed. At such high energy levels, the lamp emits a substantial amount of light, typically in a range exceeding 100 foot-lamberts or 342 candles per square meter (cd/m.sup.2).
While such light intensities may be useful in relatively high ambient light applications, in some applications such a high level of light intensity can be detrimental. For example, when high intensity fluorescent lamps are used to provide illumination for nighttime displays in automobiles, high levels of light make it difficult for the driver to view objects outside of the automobile. Consequently, it is often desirable to dim the fluorescent lamps to levels well below 1.0 foot-lambert (34 cd/m.sup.2).
To improve dimmability, a filter can be added to high intensity fluorescent lamps to block out some of the light. However, filtering can reduce the maximum light intensity of the lamps, rendering them ineffective in high ambient light environments or produce extra heat with less lumens per watt of power consumed by the lamp.
Therefore, it can be appreciated that there is a significant need for a planar fluorescent lamp having a wide illumination range. The present invention offers these and other advantages, as will be apparent from the following description and accompanying figures.